Continuing trends in electronic circuit miniaturization have led to the rapid development of circuit board surface mounted components, such as leadless "chip" capacitors, resistors, inductors, and integrated circuits. Chip resistors, in particular are very compact thick-film resistors ranging in size from smaller than about 1.0 mm by 0.5 mm (0.04 inch by 0.02 inch) to larger than about 3.0 mm by 1.5 mm (0.12 inch by 0.06 inch). Chip resistors are typically manufactured by screening and firing arrays of resistive paste materials forming resistance and connection-pad patterns on 49.5 mm by 60 mm (1.95 inch by 2.36 inch) ceramic substrates.
FIG. 1 shows an enlarged cutaway corner portion of a typical prior art substrate 10 on which is screened an array of 5,336 resistors 12 arranged in 58 columns 14 and 92 rows 16. After firing, each of resistors 12 is laser trimmed to a predetermined resistance value. Substrate 10 is then divided along score lines 17 (shown in dashed lines) into individual chip resistors and/or groups of chip resistors.
On a typical substrate, such as substrate 10, resistors 12 in columns 14 are electrically connected end-to-end by conductive pads 18 so that resistors 12 in each of columns 14 are electrically connected in series. However, resistors 12 in rows 16 are not electrically connected side-by-side.
Referring also to FIG. 2, during laser trimming, entire rows 16 or columns 14 of resistors 12 are electrically connected to a switching matrix 19 that electrically connects resistors 12 one at a time to a resistance measuring system 20 that cooperates with a laser 22 to trim each successively connected resistor 12 to its predetermined resistance value. The electrical connections to resistors 12 are made by probing conductive pads 18A and 18B with probes 24A and 24B (collectively "probes 24").
Laser trimming is a fast and accurate process that can trim 100 resistors per second to well within 5% of their predetermined values. Because laser trimming accuracy requirements often exceed 0.1%, and the predetermined resistance values may range from less than 0.1 ohm to greater than 100 Megohms, switching matrix 19 typically employs "dry-reed" relays that have an on-resistance of only about 0.1 ohms, a very high off-resistance, an insulation resistance greater than 100,000 Megohms, an actuation time less than 0.5 milliseconds, and a contact-to-contact capacitance less than 1.0 picofarad.
A typical switching matrix 19 for laser trimming an array of resistors may require several hundred relays, which even considering their stringent electrical specifications may, nevertheless, compromise measurement accuracy. For example, capacitance or insulation resistance may accumulate to an unacceptable level if too many relay contacts are connected in parallel with the resistor under test.
Skilled workers know that there are many possible ways to interconnect a particular resistor under test to resistance measuring system 20. However, the most cost-effective interconnection technique may depend on the target resistance values of the resistors under test. As a general rule, for very low resistance values (0.1 ohms to about 100 ohms), four-terminal "Kelvin connection" measurements are preferred; for intermediate values (about 100 ohms to about 100 Kohms), two-terminal measurements that add a minimum of extra resistance in series with the measured resistor are preferred; and for very high values (greater than about 100 Kohms), three-terminal "guarded" measurements are preferred. Of course, four-terminal guarded measurements are probably the most accurate for measuring any resistance value.
There are, or course, exceptions to the general rule for determining which measurement configuration to use. Groups of resistors may be interconnected so as to increase the overall measured resistance level beyond 100 ohms while still requiring four-terminal measurements. Likewise, groups of resistors may be interconnected so as to decrease the overall measured resistance below 100 Kohms while still requiring three-terminal measurements. In addition, three-terminal measurements may be required at lower than usual resistance measuring values to guard against leakage paths caused by conductive trimming debris trimmed away by the laser.
Resistance measuring system 20 typically has seven terminals that may be connected to the resistor under test. They are referred to as High-Force ("HF"), High-Sense ("HS"), Low-Force ("LF"), Low-Sense ("LS"), Guard-Force ("GF"), Guard-Sense ("GS"), and ground or common ("COM"). Also, the HF and HS terminals are often referred to as "High" terminals and the LF and LS terminals are often referred to as "Low" terminals. The force and sense terminal pairs should be connected together directly or through some acceptably low resistance relative to the resistor under test. To ensure high measurement accuracy and speed, the connection technique employed should minimize stray resistance and capacitance between High to Low terminals. One way to avoid the detrimental effects of stray resistance and capacitance is by effectively connecting such strays between the High and Guard terminals or the Low and Guard terminals.
Prior switching matrices, such as switching matrix 19, typically have either three or six relay contacts associated with each probe 24.
In the six contact per probe matrix, each of probes 24 connects to one contact of each of the relays and the other contacts connect respective to HF, HS, LF, LS, GF, and GS. The six contact per probe matrix has complete flexibility because any of probes 24 can make a two-terminal connection to one end of resistor 12, can make one-half of a Kelvin connection to one end of resistor 12, or can make a guard probe connection as required.
In the three contact per probe matrix, there are two classes of probes. In both classes, each of probes 24 connects to one contact of each of the relays. In the first class, the other contacts connect respectively to HF, LF, and GF. In the second class, the other contacts connect respectively to HS, LS, and GS. Because the force and sense terminal pairs are connected together when a resistor is connected to the measurement system, the six and three contact per probe matrices are electrically equivalent with respect to the effects of stray resistances or capacitances connected across the resistor 12 under test.
For a given number of probes, the three contact per probe matrix cannot connect to as many resistors as the six contact per probe matrix because two probes, connected together are required at each end of each resistor 12. However, the three contact per probe matrix may be implemented employing relays having two pairs of contacts per relay. One pair of contacts is used for switching the sense terminals to a probe and the other set of contacts is used for switching the force terminals to another probe.
Some relays used in prior switching matrices employ grounded internal shields between the relay coil and the dry-reed contacts. Such shields reduce undesirable coupling between the relay coil and the contacts, effectively increase the open-contact-to-contact resistance, and reduce contact-to-contact coupling because stray signal currents flow from contact to shield rather than from contact to contact.
To further reduce the effects of stray resistance and capacitance, switching matrices are often divided into groups or banks by employing bank-switching relays to disconnect the measurement system terminals from unused relays.
However, none of the above-described switch matrices can economically satisfy the ever-growing resistor array trimming requirements. For example, what is needed is a switching matrix that can economically interconnect resistance measuring system 20 via 192 probes to 48 or 96 resistors 12 in two-, three-, and four-terminal measurement configurations. The fully flexible, prior art six contact per probe matrix requires 1,152 (6.times.192) contacts (1,152 single contact or 576 dual-contact relays), without resorting to expensive bank switching. The prior art three contact per probe matrix does not meet the 96 resistor requirement.
Because each probe has contacts connected to High and Low terminals, it is possible for the stray capacitance and resistance of the many open contacts to be connected across the resistor under test, causing significant degradations in measurement speed, accuracy, and signal-to-noise ratio. Using internal relay shields reduces this problem, but at an accompanying additional cost. Bank switching can also reduce, but not eliminate, stray capacitance and resistance. Connecting the driven guard terminal to as many as necessary of the unused probes eliminates most leakage problems, but requires the simultaneous operation of many relays, which increases power consumption.
What is needed, therefore, is an inexpensive probe switching matrix for connecting a selected resistor in an array of more than 90 resistors to a resistance measuring system at a rate exceeding thousands resistors per minute without degrading the resistance measuring performance as compared to measuring a single resistor directly connected to the measurement system.